The present invention relates to a semiconductor device having a non-volatile memory cell which can be electrically erased and written; and, more particularly, the invention relates to a non-volatile semiconductor storage device, such as a flash memory, in which a threshold voltage associated with multiple-valued information comprising four values or more can be programmed in one memory cell and to a method for changing a threshold of a non-volatile cell memory. For example, the invention relates to a technique which is effectively used in a data processing system; such as a file memory system utilizing such a non-volatile semiconductor storage device.
Non-volatile semiconductor storage devices, e.g., flash memories, have been provided which can store information through injection and extraction of electrons to and from a floating gate. A flash memory includes a memory cell transistor having a floating gate, a control gate, a source and a drain. The threshold voltage of such a memory cell transistor increases as electrons are injected into the floating gate and decreases as electrons are extracted from the floating gate. The memory cell transistor stores information that depends on whether the threshold voltage is higher or lower than a word line voltage (a voltage applied to the control gate) for data readout. In this specification, a state in which the threshold voltage is lower is referred to as an "erase state" and a state in which it higher is referred to as a "write state", although this is not a limitation of the invention.
There are flash memories of this type in which information comprising four or more values is stored in one memory cell transistor. An example of articles that describe such multiple-valued memories appears on pages 48 and 49 of "Nikkei Microdevice (November, 1994 issue). Another example is Japanese Patent Laid-Open No. 297996/1997.